Best bet is to get a high level ground type with a good ground type attack. I found it easy if I chose mudkip as my starter because by then it should have evolved into marshtomp. Marshstomp is part ground type so it is immune to electric attacks, and it should know mud shot or something to take down his pokemon.
Wattson is easy as long as you have a ground type Pokemon like Marshtomp and use ground type moves like mud shot or have a grass type Pokemon like Grovyle or Breloom.
helping wattson isn't manditory so from mauville go up or north
Wattson the electric gym.
roxanne, brawly, wattson, flannery, norman.
You make sure Holmes isn't there and get a big stick.
Peekachoo
He uses electric Pokemon. His Pokemon differ between "ruby and sapphire" and "emerald".
It's the same Wattson event as in Ruby and Sapphire, but now you have to wait until after the Kyogre/Groudon event to be able to unlock the sidequest. Find Wattson in the middle of Mauville, go to new Mauville and turn off the power, and return!
In Pokemon ruby, the first electric gym is wattson's gym, it is the third badge you obtain in the game. A good strategy to beat him is to bring along a ground type, or a fire type since he uses a Pokemon called magneton, which is Electric/Steel.
You go to Mauville, which can be reached when you go avoi Brendan/May, because he/she is strong! Then just use common sense and reach Mauville, you will need to beat Wattson, electric type. Try using rare candies, or go to a day care, cuz Wattson strong and ur Pokemon will need to train cuz Wattson's pokmn.'s lv.'s are: Lv.23, Lv.22, Lv.20.
First beat the Mauville Gym leader, Wattson.
At new mauville after you beat your dad go to wattson and he'll give you directions